Technical Field
The present disclosure relates to image sensors, and more specifically to a CMOS sensor comprising pixels of dual charge-to-voltage conversion gain.
Description of the Related Art
Conventionally, a CMOS image sensor includes a plurality of pixels, each having a photodiode used in reverse mode, having its junction capacitance discharged by a photocurrent according to a received light intensity. At the end of a so-called integration period, before and after which the pixel is reset by recharging its photodiode, the photogenerated charges stored in the photodiode are transferred to a capacitive read node of the pixel. The measurement of the illumination level received by the pixel is performed by measuring the read node voltage.
The read node capacitance conditions the pixel sensitivity to illumination level variations. The lower the read node capacitance, the higher the charge-to-voltage conversion factor or gain of the pixel, in volts per electron, and conversely. Thus, a read node of low capacitance enables to relatively easily discriminate low illumination variations, especially in low illumination conditions, but may however be unable to receive all the charges photogenerated in the photodiode in case of a strong illumination. Conversely, a read node of high capacitance enables to measure high illumination levels, but does not enable to easily discriminate low illumination level variations, especially in low illumination conditions.
Image sensors where the capacitance of the read node of a pixel can be adjusted during the sensor operation have already been provided, which enables to adapt the charge-to-voltage conversion gain of the pixel, and thus its sensitivity and the extent of its dynamic range, according to the luminosity conditions.
It is however needed to improve certain aspects of existing sensors having an adjustable read capacitance.